1. Field of the Invention
The present invention relates to a method for operating a gas turbine group.
2. Discussion of Background
In a thermal power station installation, in particular an air storage installation, particular problems can arise with respect to the turbine cooling. These occur because of the very high pressure ratios of approximately 30:1 to 70:1 necessary for compatibility with an economically tolerable compressed-air reservoir. This very high pressure ratio demands, again for economic reasons, at least one reheat stage in the turbine expansion. The heat or temperature drop for the high-pressure turbine, referred to below as the HP turbine, is then substantially smaller than that of the low-pressure turbine, referred to below as the LP turbine. In an air storage gas turbine, in which the compressor is equipped with an intercooling system, compressed air is delivered to a storage cavern by the electrical machine, operated as a motor. An air/water heat exchanger cools the heated compressed air coming from the last compressor unit for the purpose of reducing its specific volume and transfers the heat into a pressurized-water accumulator installation, the corresponding shut-off elements being open during this so-called charge operation.
If electrical energy now has to be generated from the stored compressed air, the compressor group is shut down by means of a control system. The turbine group which, for example, comprises an HP turbine, an LP turbine and the electrical machine, now operated as a generator, is started by opening the corresponding closing element. This takes place initially simply by means of compressed air from the reservoir, which compressed air is preheated in the heat exchanger by the stored hot water. The production of electrical energy can be undertaken after ignition in the combustion chamber. It is, however, also possible to let the installation operate as a through-connected gas turbine by appropriate disposition of the shut-off elements and couplings. It is then possible to store some air at the same time or to extract some from the cavern.
Such an installation can no longer meet the present-day economic specifications with respect to reducing the fuel consumption and the position is aggravated by the fact that such installations are not in a position to satisfy the maximum pollutant emission figures which now have to be demonstrated. It is, in fact, correct that a waste heat boiler can be added for generating steam. The steam generated in this way can be supplied to a steam turbine group or it is directly supplied to the gas turbine group in accordance with known methods. The question of whether one or the other method of utilizing the steam is preferred depends on the duration of the turbine operation. Steam injection is correct for less than approximately 2-4 hours per day.
If such a thermal power station installation is now optimized, the hot gas temperatures corresponding to the prior art at inlet to the HP turbine produce such a high outlet temperature of the combustion gases from the HP turbine that these gases cannot be used directly as coolant for the LP turbine. For this reason, the turbine is usually cooled by cooling air which is extracted before the cavern of the air storage installation, as is usual for the cooling of turbines. In through-connected gas turbine operation, it is of course possible to avoid throttling losses by extracting the cooling air leading to the turbine at a compressor location which is appropriate with respect to pressure. Whereas, for a given mixed temperature at inlet to the turbine, the consumption of cooling air for the turbine reduces the turbine efficiency mainly because of mixing losses, the consumption of cooling air for the LP turbine has very disadvantageous effects because this cooling air bypasses the HP turbine and, therefore, does no work in it.